1. Field of the Invention
The present invention relates to an electron gun for a cathode-ray tube and, more particularly, to the structure of control and accelerating electrodes corresponding to a triode of an electron gun, which is for reducing the spot size of an electron beam in a high current region of a cathode-ray tube with high picture quality that requires high resolution.
2. Description of the Related Art
As shown in FIG. 1, a general color cathode-ray tube includes a panel 10 having a fluorescent film 11 formed on the inner side thereof, a funnel 12 formed at the rear of the panel, being combined with it, a neck 20 connected to the funnel at the back of the funnel, having an electron gun 30 placed thereinside, a deflection yoke 18 located on the outer surface of the funnel to deflect electron beams upward, downward, left and right, a shadow mask 14 placed inside the panel to perform sorting of colors, and a frame 16 supporting the shadow mask to fix it to the panel.
In the cathode-ray tube constructed as above, hot electrons are emitted from cathodes of the electron gun 30 when a video signal is applied to the electron gun 30. The emitted electrons are accelerated and focused toward the panel 10 according to a voltage applied thereto from electrodes of the electron gun. Here, the traveling path of the electron beams is adjusted by the magnetic field of a magnet set at the neck 20, and the electron beams whose traveling path was controlled scan the fluorescent film 11 formed on the inner surface of the panel 10 according to the deflection yoke 18. The deflected electron beams are subjected to color sorting while passing through lots of holes of the shadow mask 14. The color-sorted electron beams collide with the fluorescent film 11 to emit lights, presenting the video signal.
The in-line electron gun 30 placed inside the neck 20 includes control, accelerating and focusing electrodes which are located perpendicular to the traveling path of the electron beams, having a predetermined distance between neighboring electrodes, such that the electron beams generated from the cathodes can be controlled by a voltage with a predetermined level applied by the electrodes to arrive at the screen. Referring to FIG. 2, the conventional electron gun 30 for the cathode-ray tube has a tri-electrodes part 33, a front focusing lens part 34, and a main lens part 35. The tri-electrodes part 33 consists of three R, G, B cathodes 31, the control electrode G1 serving as a common grating of the three cathodes, being placed having a predetermined distance from the cathodes, and the accelerating electrode G2 located having a predetermined distance from the control electrode.
The front focusing lens part 34 is constructed of a front focusing electrode G3 placed a predetermined interval from the accelerating electrode G2, and a second accelerating electrode G4 arranged a predetermined distance from the front focusing electrode G3. The main lens part 35 includes a focusing electrode 05 located a predetermined distance from the second accelerating electrode and an anode 06 placed a predetermined distance from the accelerating electrode. The upper and lower portions of the aforementioned electrodes are inserted into a support called bead glass 39, being fixed having a predetermined interval. A shield cup 37 as a shield electrode for shielding and weakening leakage magnetism of the deflection yoke 18 is formed at the end of the anode G6.
The electron gun 30 having the above-described structure generates hot electrons when a heater 32 included in the cathode 31 is heated, and these generated electrons form electron beams (R, G, B). After the amount of emission of the electron beams is determined by the control electrode G1, the electron beams emitted are accelerated by the accelerating electrode G2 and subjected to repeated focusing and accelerating processes while passing through the front focusing lens part 34 and the main lens part 35, to be finally deflected by the deflection yoke 18 horizontally or vertically to scan the fluorescent plane 11.
The control electrode G1 of the tri-electrodes part 33 is grounded and the accelerating electrode G2 is provided with a voltage of 500-1000V The anode G6 of the main lens part 35 accepts a high voltage of 25-35 kV and the focusing electrode G5 is provided with an intermediate voltage of 20-30% of the voltage applied to the anode G6. Furthermore, the accelerating electrode G2 and the second accelerating electrode G4 are supplied with the same voltage and the front focusing electrode G3 and the focusing electrode GS are provided with the same voltage.
In general, the size of the spot at which the electron beam is focused depends on a spherical aberration generated when the electron beam passes through a passage hole of each electrode. The spot size of electron beam increases due to large spherical aberration, so that the sharpness of the electron beam is reduced, to result in deteriorate the resolution.
In the general cathode-ray tube using the in-line electron gun, three electron beams of red, green and blue are horizontally arranged in parallel. This requires a magnetism-concentrating type deflection yoke using a non-uniform magnetic field in order to converge the three electron beams on one point of the fluorescent plane 11. The distribution of the magnetic field generated by the magnetism-concentrating deflection yoke 18 has a pincushion type horizontal deflection magnetic field and a barrel type vertical deflection magnetic field, to prevent misconvergence on the fluorescent plane
This magnetic field has a two-pole component and a four-pole component. The two-pole component deflects the electron beams in a horizontal or vertical direction and the four-pole component performs not only the horizontal or vertical deflection but also prevention of misconvergence. However, these components focus the electron beams in a direction of deteriorating the resolution, that is, vertical direction, diverge the electron beams in the horizontal direction, to result in generation of astigmatism, distorting the spot of the electron beam. This distorted electron beam generates haze called phenomenon spreading of beam phase at its core and the top and bottom thereof, bringing about deterioration in the resolution at the marginal area of the screen. To solve this problem, a horizontally long groove (A) is formed at the accelerating electrode G2, as shown in FIG. 3B.
The electrodes of the electron gun have the electron beam (R, G, B) passage holes formed having a predetermined eccentric distance SI as shown in FIGS. 3A and 3B. The electron beams travel having a predetermined eccentric distance among them, and focus on one point of the fluorescent plane 11 when they proceed to the panel from the main lens part 34. In case of the conventional electron gun, the diameter of the beam passage hole of the control electrode G1 generally corresponds to 0.5-0.6 mm and that of the accelerating electrode G2 is similar to that of the control electrode G1 or increased by about 10% of thereof. Recently, the holes of the control electrode G1 and the accelerating electrode G2 have a square shape not the circular one to improve the resolution.
According to a tendency of enlargement of cathode-ray tube size and high resolution of the cathode-ray tube, a high-precision shadow mask is adopted in order to accomplish high resolution required for HDTV, for example, to realize high quality broadcasting and monitoring. The achievement of high resolution also needs a decrease in the spot size corresponding to the pixel size. To reduce the spot size of the electron beam, the effective aperture of the main lens is increased or the diameter of the beam passage hole of the control electrode G1 is reduced. However, the reduction in the diameter of the beam passage hole of the control electrode G1 shortens the distance between the cathodes 31 and the control electrode G1. This degrades activation of hot electrons to deteriorate electrical characteristic, generating a case where large quantity of current cannot be used. Furthermore, capacitance between the cathodes 31 and the control electrode G1 increases to decrease the video bandwidth of TV, deteriorating the resolution.
It is, therefore, an object of the present invention to provide an electron gun for a cathode-ray tube in which the diameter of the beam passage hole of the accelerating electrode of the tri-electrode part is larger than that of the control electrode and the control electrode is thicker than the conventional one, to reduce the electron beam spot size in a high current region, realizing picture quality with high resolution.
To accomplish the object of the present invention, there is provided an electron gun for a cathode-ray tube, comprising a tri-electrode part having a control electrode and an accelerating electrode for controlling the amount of electron beams emitted from a plurality of cathodes and accelerating the electron beams, a front focusing lens part configured of a plurality of electrodes focusing and accelerating a predetermined amount of the electron beams, and a main lens part configured of a plurality of electrodes for focusing the electron beams on a screen, in which the diameter of beam passage hole of the accelerating electrode of the tri-electrode part corresponds to 140-220% of that of the control electrode, and the thickness of the control electrode corresponds to 20-30% of the diameter of beam passage hole thereof.